SYSCTL(2)		   Linux Programmers Manual		    SYSCTL(2)



NAME
       sysctl - read/write system parameters

SYNOPSIS
       #include 
       #include 

       int _sysctl(struct __sysctl_args *args);

DESCRIPTION
       The _sysctl() call reads and/or writes kernel parameters.  For example,
       the hostname, or the maximum number of open files.   The  argument  has
       the form

	   struct __sysctl_args {
	       int    *name;	/* integer vector describing variable */
	       int     nlen;	/* length of this vector */
	       void   *oldval;	/* 0 or address where to store old value */
	       size_t *oldlenp; /* available room for old value,
				   overwritten by actual size of old value */
	       void   *newval;	/* 0 or address of new value */
	       size_t  newlen;	/* size of new value */
	   };

       This  call  does  a  search  in a tree structure, possibly resembling a
       directory tree under /proc/sys, and if  the  requested  item  is  found
       calls some appropriate routine to read or modify the value.

RETURN VALUE
       Upon successful completion, _sysctl() returns 0.  Otherwise, a value of
       -1 is returned and errno is set to indicate the error.

ERRORS
       EFAULT The invocation asked for the previous value  by  setting	oldval
	      non-NULL, but allowed zero room in oldlenp.

       ENOTDIR
	      name was not found.

       EPERM  No  search  permission for one of the encountered "directories",
	      or no read permission where oldval was  non-zero,  or  no  write
	      permission where newval was non-zero.

CONFORMING TO
       This  call  is  Linux-specific,	and  should  not  be  used in programs
       intended to be portable.  A sysctl() call has  been  present  in  Linux
       since  version  1.3.57.	 It  originated in 4.4BSD.  Only Linux has the
       /proc/sys mirror, and the object naming schemes	differ	between  Linux
       and 4.4BSD, but the declaration of the sysctl() function is the same in
       both.

NOTES
       Glibc does not provide a wrapper for this system call;  call  it  using
       syscall(2).

       Or  rather... dont call it: use of this system call has long been dis
       couraged, and it is so unloved that it is  likely  to  disappear  in  a
       future  kernel  version.   Remove  it  from  your programs now; use the
       /proc/sys interface instead.

BUGS
       The object names vary between kernel versions, making this system  call
       worthless for applications.

       Not all available objects are properly documented.

       It  is  not  yet  possible  to  change  operating  system by writing to
       /proc/sys/kernel/ostype.

EXAMPLE
       #define _GNU_SOURCE
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       int _sysctl(struct __sysctl_args *args );

       #define OSNAMESZ 100

       int
       main(void)
       {
	   struct __sysctl_args args;
	   char osname[OSNAMESZ];
	   size_t osnamelth;
	   int name[] = { CTL_KERN, KERN_OSTYPE };

	   memset(&args, 0, sizeof(struct __sysctl_args));
	   args.name = name;
	   args.nlen = sizeof(name)/sizeof(name[0]);
	   args.oldval = osname;
	   args.oldlenp = &osnamelth;

	   osnamelth = sizeof(osname);

	   if (syscall(SYS__sysctl, &args) == -1) {
	       perror("_sysctl");
	       exit(EXIT_FAILURE);
	   }
	   printf("This machine is running %*s\n", osnamelth, osname);
	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       proc(5)

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux				  2007-09-22			     SYSCTL(2)